Whenever Peach is kidnapped she is alone with Bowser for a really long time and I'm sure that the both of them get really lonley, because Mario takes a long time to finally find them. I'm gussing the first time bowser took peach she was raped, but peach probably started to love it which is why she keeps getting "kidnapped". Because a giant turtle/dragon probably has a bigger penis than a fat italian guy that is the size of a four year old child. Heres what really drives my point home. Mario has no kids, but there is a little bowser jr. running around.

I see there's 2 ways how this can go.
1. Bowser as a giant has an enormous manthing and if he were to sex Peach up, she should be dead by now.

2. Bowser as a turtle has only a very small penis unsuitable for mating. That's why all his kids are artificial.

My answer to why all these kidnappings?
Bowser wants to eat!
That's why he invades mushroomkingdom. He stores the best of the mushrooms he finds in back up castles. That's why Mario always finds mushrooms first.
He then takes the princess into a furnace like room and like in Hansel and Gretel tries to get the peach fatter and fatter till she's right to eat. Like the story goes, Mario comes to face Bowser and save the princess. Doing so he pushes Bowser in the stove to fry, just as Gretel did in the tale.
The tale of Mario is thus nothing but an inventive version of Hansel and Gretel.